In general, a thermally sensitive recording medium having a thermally sensitive recording layer whose main components are colorless or pale colored electron donating leuco dye (hereinafter shortened to dye) and electron accepting color developing agent (hereinafter shortened to color developing agent) that develops color when heated with a dye is practically used widely. A thermal printer in which a thermal head is built in is used to record images on it. This thermally sensitive recording method has advantages that, noiseless at recording process, developing process and fixing process are not necessary, maintenance free, an apparatus is relatively cheap and compact and a obtained color is very clear, therefore, is widely applied in the field of a facsimile, a terminal printer of computer, an automatic ticket vendor, a recorder for measuring instrument or a handy terminal that is used in outdoor. As uses of this thermally sensitive recording medium, besides above mentioned output paper for various instruments, the thermally sensitive recording medium are becoming to be used in the field of a paper for note to which good preservability is required. Along with the diversification of uses, recording instrument itself is disversified and becoming high performance (high speed, compact), accordingly, for the thermally sensitive recoding medium, a product which has higher color developing sensitivity and can obtain high quality recorded image in all range from lower printing density to higher printing sensitivity.
In general, as the method to improve color developing sensitivity and quality of developed image, a method to improve smoothness of an under coating layer formed on the surface of a thermally sensitive recording medium or between a substrate and thermally sensitive recording layer by a super calendar is well known.
In Patent Document 1, for the purpose to provide a thermally sensitive recording material which is excellent in dot reappearance ability (quality of image), a technique to laminate first intermediate layer and second intermediate layer in order between substrate and thermally sensitive color developing layer, wherein Ohken smoothness of the first intermediate layer is 700 seconds or more and density of the second intermediate layer is 0.1 or less is disclosed. In Patent Document 2, a technique to contain scale shape silica particles in an intermediate layer formed between a substrate and a thermally sensitive recording layer is disclosed. However, in a case when the smoothness of the thermally sensitive recording medium is improved by means of super calendar, although high quality recorded image can be obtained, porosity (abiabatic ability) of the layers which construct a thermally sensitive medium is deteriorated by calendar pressure. Accordingly, thermal efficiency at the thermally sensitive layer is deteriorated and high color developing sensitivity can not be obtained. Further, recently, since the required level for quality of image is becoming higher, sufficient quality can not be obtained by only improving the smoothness of the surface of the thermally sensitive recording medium or the under coating layer.
Further, along with the diversification of uses, a thermally sensitive recording medium to which fixed informations such as logo mark or ruled lines are previously printed by a common printing method such as offset printing is becoming to be used more frequently, accordingly, a high lustrous thermally sensitive recording medium having aptitude for a common printing, especially high lustrous aptitude for a printed part is required.
In Patent Document 3, for the purpose to improve aptitude for a common printing and lustor of thermally sensitive recording medium, an art characterized to contain aluminum hydroxide in a thermally sensitive layer is disclosed and in Patent Document 4, an art to form a protecting layer for the purpose to obtain high lustrous thermally sensitive recording medium is disclosed. However, when a protection layer is formed, since heat conduction to a thermally sensitive recording layer becomes slow, not only disadvantages such as deterioration of sensitivity, deterioration of printing speed or increase of printing energy cause, but also a problem of production cost rising causes because numbers of coating layer are increased.
Further, for the purpose to obtain high color developing sensitivity, there is an art to blend a silica having high oil absorbing ability in a thermally sensitive recording medium and to fix fused dye and color developing agent, however, when silica is blended, since lustrous of a thermally sensitive recording medium is deteriorated, problem that high printing lustrous can not be obtained causes. As mentioned above, at the present time, a thermally sensitive recording medium having high color developing sensitivity and characterized that lustrous of printed part is high when printed by a common printing method such as an offset printing, is not obtained yet.
Furthermore, since basic leuco dye and color developing agent contained in a thermally sensitive recording medium can be easily dissolved in various solvent, the thermally sensitive recording medium has a problem that the developed color density is deteriorated when a developed image is contacted with chemicals such as plasticizer contained in an ink of water mark pen or oily mark pen or an adhesive. In Patent Documents 5 and 6, for the purpose to overcome said defect, an art to form a protecting layer containing a pigment and a resin as main components on a thermally sensitive recording medium is disclosed.
However, recently, uses of a thermally sensitive recording medium are extending widely, for example, for various tickets, receipt, label, for ATM use of bank, for inspection of city gas or water supply or for a note of cycle race or horse race. Therefore, severe requirement for feature of the thermally sensitive recording medium is rising, which is not required to a conventional thermally sensitive recording medium. Since these cases of use are mainly outdoor use, the function of an over coating layer, which can endure practical use in more severe environment compared with the conventional use such as rain, water, humid, sun shine or in a car of summer, becomes necessary. However, when such kind of protecting layer is formed, although stability can be improved, above mentioned disadvantage such as deterioration of sensitivity, deterioration of printing speed or increase of printing energy are caused, further, large friction is caused between pigment such as silica contained in a protecting layer with a head, a problem of abrasion of a head is caused when continuously used for long time.
(Patent Document 1) JP 2000-108528 A publication
(Patent Document 2) JP 2002-137542 A publication
(Patent Document 3) JP Patent No. 2786912
(Patent Document 4) JP H09-156222 A publication
(Patent Document 5) JP S48-30437 A publication
(Patent Document 6) JP S48-31958 A publication